A step automatic transmission is known from US 2003/0087720 A1 with a planetary set and pressure-activated transmission shifting elements. The transmission shifting elements are constructed as two switchable couplings which use a common outer disc carrier to accept the outer discs of the couplings. The common outer disc carrier shows a pot-shaped, half cross-sectional geometry whose symmetry axis coincides with the long axis of the transmission input shaft. As a result, the outer disc carrier forms a coupling area within which the disc packet and the servo device of both couplings are positioned.
The two disc packets of the mentioned couplings, which will be examined more closely here, are positioned immediately next to each other in an axial and radial direction. In addition, the servo devices act in such a way on both coupling packets, that the closing direction of both couplings is equal and points away from the pot base of the outer disc carrier. For that reason, the pressure areas of both servo devices are axially directly next to each other.
The piston of the servo device which activates the second disc packet located away from the pot base is axially located between the pistons of the servo device to activate the first disc coupling near the pot base, as well as the pot base of the outer disc carrier. In addition, the piston to activate the first disc packet near the pot base is positioned between the piston for the second disc packet away from the pot base and the disc packet near the pot base.
It should also be noted, that the inner discs of both couplings of this known automatic transmission are carried by two separate inner disc carriers.
By means of this, construction results that the piston to activate the second disc packet away from the pot base axially separates the two pressure areas of both servo devices.
Another characteristic of the transmission is that only the servo device to activate the first disc packet near the pot base is associated with a space for a dynamic, activation pressure compensation. In the process, the mentioned piston which activates the first disc packet near the pot base separates the pressure area for this piston as well as the pressure compensation area axially from each other.
In this transmission the fact that the pressure in the pressure area of the servo device to activate the first coupling near the pot base influences the pressure in the pressure area of the servo device to activate the second coupling away from the pot base and vice versa can be viewed as disadvantageous.
In addition, the fact that the piston must activate the second coupling away from the pot base axially penetrating the outer discs of the first coupling can be viewed as expensive to build, away from the pot base. For that reason, the outer discs of the first coupling near the pot base are equipped with axial penetration openings in the area of their outer diameter through which the mentioned piston is guided. Moreover the outer discs of the first disc coupling must be built comparatively long in a radial direction which then results in an enlarged diameter for the transmission.